MessageLabs attributes the drop to a production break from the Rustock, Lethic and Xarvester botnets. Rustock has all but shut down while the fiendish hackers behind the Lethic and Xarvester have also gone quiet.

By contrast, two other significant sources of spam - Gheg and Cutwail - are pumping out junk mail at much the same volumes as ever, as a graph from MessageLabs explains.

MessageLabs reckons the drop is unlikely to be anything more than a temporary respite from the (almost) relentless torrent of nuisance email messages touting penis pills, "investment opportunities", smut and other assorted tat.